Fix #8803: Only auto-remove signals when rail can be built (#8904)

src/rail_cmd.cpp

@@ -459,6 +459,10 @@ CommandCost CmdBuildSingleRail(TileIndex tile, DoCommandFlag flags, uint32 p1, u
 			if (ret.Succeeded()) ret = EnsureNoTrainOnTrack(tile, track);
 			if (ret.Failed()) return ret;
 
+			ret = CheckRailSlope(tileh, trackbit, GetTrackBits(tile), tile);
+			if (ret.Failed()) return ret;
+			cost.AddCost(ret);
+
 			if (HasSignals(tile) && TracksOverlap(GetTrackBits(tile) | TrackToTrackBits(track))) {
 				/* If adding the new track causes any overlap, all signals must be removed first */
 				if (!auto_remove_signals) return_cmd_error(STR_ERROR_MUST_REMOVE_SIGNALS_FIRST);
@@ -472,10 +476,6 @@ CommandCost CmdBuildSingleRail(TileIndex tile, DoCommandFlag flags, uint32 p1, u
 				}
 			}
 
-			ret = CheckRailSlope(tileh, trackbit, GetTrackBits(tile), tile);
-			if (ret.Failed()) return ret;
-			cost.AddCost(ret);
-
 			/* If the rail types don't match, try to convert only if engines of
 			 * the new rail type are not powered on the present rail type and engines of
 			 * the present rail type are powered on the new rail type. */